is here a css-guru? In my opinion we need a custom style for the TinyMCE Dialog-Buttons. The buttons are so different to the Ext.Buttons and a little bit ugly when rendering inside a Dialog...

Dumbletore, nothing really "guruish" is necessary for this. Take o2k7 skin as a base and with help of Firebug twik it. Firebug shows even line in css file where specific class resides. So it's very simple to find things to be modified. Most likely, you'll need only to modify toolbar backgrounds. Dialog also could be modified to be more Ext-like.